This invention relates to microwave filters. In particular, this invention relates to cavity filters formed by cylindrical cavities possibly including RF resonators located within the cavities to tune the filter's response.
Prior art 1/2-wavelength microwave cavity resonators are typically constructed with a ratio between the outer and inner conductors of 3.59 to 1 for optimum Q. Power loss in these filters is a significant problem and is principally attributed to ohmic losses in the inner conductor of the filter. The size of the inner conductor may be increased to reduce ohmic losses. Increaing the inner conductor, however, must be accompanied by an increase in the size of the outer conductor of approximately 3.59 times that of the inner conductor to obtain a Q improvement. On the other hand, ohmic losses in a cavity resonator could be substantially reduced by usage of superconducting materials including new high temperature superconducting materials such as Ytrium-Barium-Copper-Oxide (YBC).
In a normal or typical prior art cavity filter the center conductor is usually 1/4 wave length long requiring a low loss junction between the center conductor and one end, top or bottom, of the cavity where connection is ordinarily made. Since a 1/4 wave length center conductor requires a direct physical contact, using a 1/4 wave length center conductor made of a superconductor would pose serious electrical and mechanical connection problems due to the direct contact with a non-superconducting material forming the outer conductor of the cavity. A microwave cavity filter having at least superconductors in the inner conductor of the cavity that does not require direct contact with non-superconducting materials that substantially reduces ohmic losses would be an improvement over the prior art.